1. Field of the Invention
The present invention relates to a surface acoustic wave device, a shear bulk wave transducer, and a longitudinal bulk wave transducer, each having a ZnO film with a low resistance disposed on a substrate so as to function as an electrode.
2. Description of the Related Art
The Japanese Unexamined Patent Application Publication No. 8-162881 discloses a surface acoustic wave filter having a ZnO layer as an electrode in which the resistance is decreased by doping with an impurity. According to the method of producing a surface acoustic wave filter described in the above-identified Japanese Unexamined Patent, a ZnO layer is provided on a substrate, and ions are implanted in the ZnO layer, so that the impurity dopes the ZnO in an area near the boundary between the substrate and the ZnO layer, and thus, a ZnO layer with a low resistance is provided. Moreover, the above-mentioned ZnO layer with a low resistance is utilized as an interdigital electrode. That is, the surface acoustic wave device includes an interdigital electrode made of ZnO having a low resistance disposed on the substrate, and a ZnO layer, as a piezoelectric layer, is provided on the interdigital electrode.
On the other hand, the Japanese Unexamined Patent Application Publication No. 8-228398 discloses a shear bulk wave transducer using an ZnO layer with a low resistance as well.
A ZnO layer with a low resistance, arranged to function as an electrode, is disposed on a substrate made of an R-plane [(01{overscore (1)}2) plane] sapphire. A second ZnO layer with a high resistance is arranged as an epitaxial film on the first ZnO layer. An electrode is disposed on the second ZnO layer.
In both of the surface acoustic wave filter and the shear bulk wave transducer described in the Japanese Unexamined Patent Application Publication No. 8-162881 and the Japanese Unexamined Patent Application Publication No. 8-228398, the ZnO layer with a low resistance containing an impurity is arranged so as to be in contact with the ZnO layer the defines a piezoelectric film, and is used as an electrode.
Referring to the ZnO layer with a low resistance in the surface acoustic wave filter described in the Japanese Unexamined Patent Application No. 8-162881, the specific resistance xcfx81 is low, that is, about 1xc3x9710xe2x88x923 xcexa9xc2x7cm. However, the specific resistance is too high from the standpoint of a material for defining an interdigital electrode. Al and Au which are ordinary electrode materials for defining interdigital electrodes have a specific resistance of about 2xc3x9710xe2x88x928 xcexa9xc2x7cm. For this reason, in the surface acoustic wave filter described in the Japanese Unexamined Patent Application Publication No. 8-162881, the interdigital electrode has a high resistance, and the insertion loss is large. Thus, the surface acoustic wave filter is unsuitable for practical use.
In addition, impurities doping by ion implanting likely causes a large stress in a film to be doped, and it is difficult to implant the impurities evenly in the film, which degrades the qualities of the doped film. Moreover, an ion implanting apparatus is relatively expensive.
On the other hand, the Japanese Unexamined Patent Application Publication No. 8-228398 discloses a process of forming a ZnO layer with a low resistance on a substrate by sputtering, using a target produced by doping a ZnO molding with an impurity such as Al. According to this process, the impurity can be distributed more uniformly, and the specific resistance xcfx81 can be effectively reduced, as compared with the method of producing the surface acoustic wave filter described in the Japanese Unexamined Patent Application Publication No. 8-1628881.
It should be noted that the Japanese Unexamined Patent Application Publication No. 8-228398 discloses only the shear bulk wave transducer in which the ZnO layer with a low resistance, the second ZnO layer with a high resistance, and the electrode are formed on the support made of the R-plane sapphire. No reference to a surface acoustic wave device using an interdigital electrode is made.
In order to overcome the problems and shortcomings of the prior art described above, preferred embodiments of the present invention provide a surface acoustic wave device including an electrode defined by a ZnO layer with a low resistance having an impurity doped therein, and having a greatly reduced insertion loss.
In addition, preferred embodiments of the present invention provide a shear bulk wave transducer and a longitudinal bulk wave transducer each of which uses different types of substrate materials and a ZnO layer with a low resistance.
According to a first preferred embodiment of the present invention, a surface acoustic wave device includes a substrate, an electrode disposed on the substrate and having a specific resistance of up to about 1 xcexa9xc2x7cm, and including a ZnO layer with a low resistance being doped by an impurity, a piezoelectric film arranged as an epitaxial film on the electrode including the ZnO layer, and having a specific resistance xcfx81 of at least about 106 xcexa9xc2x7cm and a lattice constant within approximately xc2x120% of that of ZnO, and an interdigital electrode disposed on the piezoelectric film.
Preferably, the substrate is made of one of a material selected from the group consisting of R-plane, sapphire having a plane parallel to the c axis, Z-cut quartz, (111), (100), or (011) silicon, about 30xc2x0 to 42xc2x0, rotated Y-plate LiNbO3, about 30xc2x0 to 42xc2x0 rotated Y-plate LiTaO3, and planes equivalent thereto.
Also preferably, the doping impurity is added at a ratio of approximately 3% to 5% by weight based on 100% by weight of ZnO.
Moreover, preferably, as the impurity, an atom or molecule belonging to group IIIA, IIIB, VA, or VB, and more preferably, at least one selected from the group consisting of Al, Ga, V, Nb, and As is used.
According to a second preferred embodiment of the present invention, a shear bulk wave transducer includes a substrate made of a material selected from the group consisting of approximately 30xc2x0 to 42xc2x0 rotated Y-plate LiNbO3, and approximately 30xc2x0 to approximately 42xc2x0 rotated Y-plate LiTaO3, an electrode disposed on the substrate and having a specific resistance xcfx81 of up to about 1 xcexa9xc2x7cm, and including a ZnO layer with a low resistance having as an impurity an atom or molecule belonging to group IIIA, IIIB, VA, or VB, doping the ZnO, a piezoelectric film arranged as an epitaxial film on the electrode including the ZnO with a low resistance, and having a specific resistance xcfx81 of at least about 106 xcexa9xc2x7cm and a lattice constant within approximately xc2x120% of that of ZnO, and an electrode disposed on the piezoelectric film.
Preferably, as the impurity, at least one material selected from the group consisting of Al, Ga, V, Nb, and As is used.
According to a third preferred embodiment of the present invention, a longitudinal bulk wave transducer includes a substrate made of a material selected from the group consisting of c-plane sapphire, sapphire having a plane parallel to the c axis, (111), (110) or (011) silicon, and Z-cut quartz, and planes equivalent thereto, an electrode disposed on the substrate, having a specific resistance xcfx81 of up to about 1 xcexa9xc2x7cm, and made of a ZnO film with a low resistance having as an impurity an atom or molecule belonging to group IIIA, IIIB, VA, or VB, doping the ZnO, a piezoelectric film arranged as an epitaxial film on the electrode including the ZnO with a low resistance, and having a specific resistance xcfx81 of at least about 106 xcexa9xc2x7cm, and a lattice constant within approximately xc2x120% of that of ZnO, and an electrode disposed on the piezoelectric film.
Preferably, as the impurity, at least one material selected from the group consisting of Al, Ga, V, Nb, and As is used.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention.